The present invention relates to a method of forming a silicon oxide film, a method of production of a semiconductor memory device, and a computer readable storage medium.
One type of semiconductor device, a flash memory, has a small size of cells and can be made large in capacity and is also superior in shock resistance, so has been rapidly growing in demand in recent years. As a typical system used in a flash memory, it is known to form an ONO (oxide-nitride-oxide) structure insulating film between a control gate electrode and a floating electrode and inject electrons to the floating gate electrode paired with the control gate electrode or remove them so as to write and erase data.
Further, among flash memories, ones having a multilayer structure called the SONOS (silicon-oxide-nitride-oxide-silicon) type or the MONOS (metal-oxide-nitride-oxide-silicon) type are known. In these types of flash memories, a silicon nitride film (nitride) sandwiched between the silicon dioxide films (silicon-oxide) holds information as a charge storage layer. That is, voltage is applied between a semiconductor substrate (silicon) and a control gate electrode (silicon or metal) to inject electrons into the silicon nitride film of the charge trapping layer to store data or to remove electrons stored at the silicon nitride film to erase data.
A flash memory has superior characteristics as a storage medium as explained above, but there is the problem that there are limits to the number of times data can be written. The tunnel oxide film of a flash memory has to maintain its insulating ability over a long period in order to hold charges injected into the charge storage layer (the above floating gate electrode or silicon nitride film etc.) However, if repeating a data write operation or erase operation applying voltage to run electrons through the tunnel oxide film hundreds of thousands of times, the tunnel oxide film will deteriorate and leakage current will increase (SILC: Stress Induced Leakage Current) and therefore the function as an insulating film will drop. Such deterioration of the tunnel oxide film has become a factor limiting the product lifetime of flash memories and lowering reliability.
A tunnel oxide film usually is a silicon dioxide film formed by oxidation of the surface of silicon. As a method of oxidizing silicon, in general thermal oxidation using a WVG (water vapor generator), thermal radical oxidation such as ISSG (in-situ water vapor generation), and other thermal oxidation and plasma oxidation utilizing plasma for oxidation are known, but in the case of a tunnel oxide film, for the purpose of giving a flash memory high reliability, a high quality insulating film of a thermal oxide film (including thermal radical oxide film) is being used. A thermal oxide film is formed by thermal oxidation of the silicon surface at high temperature such as usually 800° C. or more than 800° C. However, even if using a thermal oxide film, the tunnel oxide film continues to degrade when repeating a data write operation or erase operation, so improvement has been sought.
Japanese Patent Publication (JPA) No. 2004-356528 proposes the method of improving the insulation performance of a gate insulating film to reduce the leakage current by treating the insulating film for improvement by oxygen plasma, then treating it for improvement by thermal annealing. However, JPA 2004-356528 covers improvement of an insulating film formed by CVD. In general, compared with a silicon oxide film obtained by CVD, a thermal oxide film obtained by thermal oxidation of silicon is dense and extremely high in quality, so almost no attention has been paid up to now to further improving the performance of a thermal oxide film.
As indicators of the reliability of an insulating film, two parameters obtained from the TDDB (time dependent dielectric breakdown) are known. One is the Qbd characteristic expressing the total charge until dielectric breakdown when giving electrical stress to an insulating film, while the other is the Rd characteristic expressing the amount of voltage shift with respect to the electrical stress (degradation speed) (refer to Z. Lu et al., IWDTF, 123 (2008).) In evaluating the reliability of the tunnel oxide film of a flash memory as well, it is believed necessary to consider said two parameters.